[Ru(bipy)(3)](2+) and [Os(bipy)(3)](2+) chromophores as sensitisers for near-infrared luminescence from Yb(iii) and Nd(iii) in d/f dyads: contributions from Förster, Dexter, and redox-based energy-transfer mechanisms.

The complexes and contain M(bipy)(3) chromophores with a pendant aza-18-crown-6 macrocycle for binding of lanthanide(iii) ions. The photophysical properties of the adducts . and ., prepared by addition of excess Ln(NO(3))(3) (Ln = Nd, Yb) to solutions of and in MeCN, were examined using time-resolved and steady-state luminescence methods. Whereas does not act as an energy-donor to Yb(iii), it will transfer energy to (and generate sensitised near-infrared luminescence from) Nd(iii) with a Ru(ii)-->Nd(iii) energy-transfer rate constant of 6.8 x 10(6) s(-1). In contrast, is quenched by both Yb(iii) and Nd(iii), but with faster energy-transfer to Yb(iii) (2.6 x 10(7) s(-1)) than to Nd(iii) (1.4 x 10(7) s(-1)). Thus d --> f energy transfer is in both cases faster for Os(ii) than for Ru(ii), but the relative ability of Nd(iii) and Yb(iii) to act as energy-acceptors is inverted from . to .. Reasons for this are discussed with reference to contributions from the Förster and Dexter mechanism for energy-transfer in . and ., using calculated spectroscopic overlap integrals coupled with molecular modelling to estimate inter-chromophore separations. The particular effectiveness of Os(ii) --> Yb(iii) energy-transfer in . is explained in terms of the Horrocks redox mechanism involving an initial *Os(ii) --> Yb(iii) photoinduced electron transfer step generating an Os(iii)/Yb(ii) state, which is shown to be marginally favourable for ., but not for . in which the Ru(bipy)(3) unit is a poorer excited-state electron-donor by about 0.1 eV.